\begin{tikzpicture}
	\tkzDefPoints{0/0/b1, 3/0/b2, 0/1/a1, 2/1/a2}
	\tkzDrawSegments[xianduan={below=0pt}](b1,b2  a1,a2)
	\tkzLabelSegment[above](a1,a2){$a$}
	\tkzLabelSegment[above](b1,b2){$b$}
\end{tikzpicture}

